Chlorine is useful as a raw material of vinylchloride, phosgene, etc., and is known to be obtained by oxidation of hydrogen chloride. Chlorine used in such use is generally desired to have a high-purity quality of 99% by volume or more. An example of the methods of producing high-purity chlorine includes a method of producing chlorine by a hydrochloric acid oxidation process described in Japanese Patent Laying-Open No. 2005-306715 (Patent Document 1). The above-described Patent Document 1 discloses that chlorine with a desired quality (purity) is obtained by purifying a mixed gas containing chlorine obtained by performing an oxidation reaction of gas containing hydrogen chloride with oxygen in a distillation column and separating oxygen and carbon dioxide that accompanies with the mixed gas. The quality of chlorine is determined by the cooling temperature, the bottom temperature, the number of steps of the distillation column, etc.
Since the inside of the system is filled with an inert gas, air, etc. during the start-up of the chlorine oxidation process in the Patent Document 1, the purity of the chlorine purified in the distillation column decreases by mixing the inert gas and air again. Therefore, it is necessary to discharge the inert gas and air in the system during the start-up. Accordingly, the chlorine purified in the distillation column is normally discharged outside of the system through a pipe connected to the outside of the system without sending it out to a pipe that feeds product chlorine.
However, since purging such gas outside of the system during the start-up is to discharge the chlorine purified in the distillation column from the inside of the system, there is a problem that efficiency of producing chlorine decreases.
Further, it is necessary to detoxify chlorine from the gas containing chlorine by neutralizing with an alkaline solution before purging it to the outside of the system. Therefore, there is a problem that the alkaline solution is consumed during the start-up.